1. Field of the Invention
The present invention relates to a fuel injection control system for a direct injection type internal combustion engine. The system controls the fuel injection start timing and fuel injection duration to occur any time during an extended time interval from at least the beginning of a suction stroke to the end of a compression stroke to achieve fuel economy at low and middle engine loads and to achieve a high power output characteristic at a high engine load.
2. Description of the Prior Art
The direct injection type internal combustion engine has a construction wherein a given amount of fuel is injected directly into an engine cylinder and the fuel spraying characteristics and air stream within the engine cylinder are precisely controlled to form a region of combustible air-fuel mixture in the vicinity of an ignition plug, thereby facilitating stable combustion of the air-fuel mixture at every mixture ratio the air-fuel mixture.
It is necessary to control the fuel spraying characteristic within each engine cylinder precisely. In addition, it is necessary to control the timing of fuel injection start and fuel injection end (fuel injection duration) more extensively than in a conventional internal combustion engine of the indirect injection type since in the direct injection type internal combustion engine it is difficult to carry out precombustion of the air-fuel mixture which always occurs in a conventional internal combustion engine wherein the fuel is injected into intake manifolds behind intake valves of the engine cylinders.
For example, it is desirable that in a region of low engine load, the end of fuel injection through a fuel injection valve be timed immediately before the spark timing, i.e., at substantially the end of the compression stroke to avoid misfire and that in a region of high engine load, the end of fuel injection be timed at an earlier stage, e.g., at substantially the end of a suction stroke (when the piston arrives at bottom dead center).
However, in conventional fuel injection control systems fuel injection cannot flexibily be carried out over a wide range of times, including the suction and compression strokes of one engine cycle since the conventional fuel injection control systems only inject during the suction stroke of each engine cylinder according to a reference angle pulse produced whenever an engine crankshaft has rotated through 180.degree. (180.degree. in the case of four cylinder engine, 720.degree..div.cylinder number in general), and the conventional control systems cannot inject through more than one fuel injection valve. That is to say the conventional control system cannot overlap the fuel injection through one of the fuel injection valves while any other fuel injection valve is injecting.
Therefore, it is difficult to reduce fuel consumption through combustion of an extremely lean air-fuel mixture under a low engine load, since extensive control of the timing of fuel injection start and fuel injection end (fuel injection duration) cannot be effected with conventional control systems.